Genetic

Management of

Small Populations:

the Special Case of

Feral Horses

By: Oliver A. Ryder 

Center for the Reproduction of

Endangered Species, Zoological

Society of San Diego

The sixth in a series of 13,

Session 2

Background

Only the Plains zebra, among

all the Equidae, can be counted as a wild species that is not

endangered. The Przewalski’s

horse (Equus przewalskii),

the African wild ass (Equus

africanus somaliensis), the

Asiatic wild asses (E. kiang;

E. hemionus onager; E.

hemionus luteus; E. hemionus

khur), Grevy’s zebra (E. grevyi)

and the Mountain zebras (E.

zebra zebra; E. zebra hartmannae) are all endangered species

(see Resource Note #24).

Donkeys and horses, the

domestic equids, thrive in the

presence of human culture,

contrary to the situation pertaining for populations of

wild equids.

Discussion

Conservation efforts for wild

equids include establishment

of protected areas for surviving

populations, such as Samburu

National Park in Kenya and

Cape Mountain Zebra

National Park in South Africa.

Often, habitat alterations

are undertaken in order to

support these native species,

including management

action to ensure access to

water resources. Ex situ conservation of equid gene pools

involve intensive management of small populations

through programs like the

Species Survival Plans (SSP)

of the American Zoo and

Aquarium Association. The

goal of the structured breeding programs of SSP is to

preserve target amounts of

genetic variation for defined

periods of time (typically

90-95% of average heterozygosity over 100-200 years).

Equalizing founder contribution and family size maximizes

the genetically effective size of

these managed populations.

Domestic horses have become

feral at numerous times and

at numerous places in the

world, establishing expanding populations that have

produced a variety of environmental impacts. Feral

horse populations have tended to be managed first for

demographic considerations

(carrying capacity, number

of individuals removed).

Founder effect, natural

selection, genetic drift, and

gene flow through introduction of new individuals

into relatively isolated

breeding populations serve

as the major features influencing the gene pool of

feral horse populations.

Selective forces acting on feral

horses include the impacts of

RESOURCE NOTES

NO. 28 DATE 07/24/00

Range

Wild Horse andBurro Program
the natural environment as

well as anthropogenic influences, such as removal of

individuals from the herd.

Conclusion

Assuming that management

of populations of feral horses

requires identification of the

maximum number of individuals to occupy individual

management units (carrying

capacity), managers are

faced with decisions regarding impacts of intervention

on the gene pool. Differing

scenarios for removal of individuals in order to maintain

populations below carrying

capacity produce different

impacts on the population’s

gene pool over time, especially under circumstances in

which migration of individuals

between populations is rare.

Modern methods of genetic

analysis applied to horses that

have been removed from feral

populations can shed insights

into population structure and

help constrain alternatives for

population management.

Contact

Dr. Oliver A. Ryder, PhD.

Adjunct Professor of Biology

University of California 

San Diego, Center for

Reproduction of Endangered

Species, Zoological Society of

San Diego 

P.O. Box 551 

San Diego, California

92112-0551

phone 619-557-3950

fax 619-557-3958

e-mail: oryder@ucsd.edu.

Range

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Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

and opinions expressed herein do not

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